Determination of secondary electron spectra from insulators

Abstract

Summary:A new technique for the determination of secondary electron (SE) spectra of insulators in a scanning electron microscope environment is presented. It is based on a capacitatively coupled charge measurement by subjecting the insulating film to a controlled pulsed electron beam. With the use of a planar grid analyzer configuration, an algorithm is used to estimate the SE spectrum based on normalized values of the S-curve obtained. Secondary electron spectra from several insulating materials employed in integrated circuit manufacturing, including silicon nitride (Si 3 N 4 ), AZ 1350J photoresist, and pyralin polyimide, have been measured.

Experimental secondary electron spectra under SEM conditions

“…(Note, however, that if these spectra are compared in the N ( E ) vs. E format these differences are minimized and visually the level of agreement is excellent in all cases.) The differences that exist between these two sets of data, and between the spectra presented here and the examples shown by Bleloch (1989), Whetten & Laponsky (1957) and Yong & Thong (2000), can probably be attributed to environmental factors, but also to the fact that different spectrometers and collection geometries were employed in each case. The energy spectra of SE emission is different at different take‐off angles from the surface because of the effects of SE transmission and reflection at the surface potential barrier (Keneko, 1990).…”

“…It can be seen that the peak energy is typically about 5–6 eV, with the lowest being for silicon (4 eV) and the highest (11 eV) for Rh and Sb. These values are significantly higher than the 2–3 eV typically measured for clean samples under UHV conditions (Modinos, 1985), from partially cleaned samples examined at UHV (Whetten & Laponsky, 1957; Bleloch, 1989), and from insulators examined using a low duty‐cycle pulsed beam to eliminate charging (Yong & Thong, 2000). Because a thin layer of surface contamination acquires a positive charge during irradiation in the microscope the lowest energy secondaries passing through the film will be those most strongly retarded, leading to an upward shift in the apparent peak position.…”

“…where δ( E ) is the SE yield at the emitted energy E measured with respect to the Fermi level of the sample, k is a normalization constant, and φ is the work function. Although this model only strictly applies to a free electron metal, the fact that the result is given in an analytical form with only one free parameter makes it a convenient template for the evaluation of spectra (Nickles et al ., 2000; Yong & Thong, 2000). Figure 8(a–c) compare the N ( E ) spectra for Al, Ge and a bronze alloy, respectively, with the Chung and Everhart expression ().…”

“…The advent of the scanning electron microscope (SEM), which relies on secondary electrons for its most popular mode of imaging, ensured continuing interest in the properties of SE although again most experimental work has been concentrated on the yield variation with energy because of its importance in understanding the charging behaviour of insulating samples (Joy & Joy, 1999; Joy, 2003). However, many users of the SEM have begun to recognize that a knowledge of the energy spectrum of the secondaries would be of value in interpreting images and in designing more efficient and contrast‐specific detector systems (Bleloch, 1989; Sakai et al ., 1998; Yong & Thong, 2000).…”

Quantitative material analysis using secondary electron energy spectromicroscopy

“…There is therefore, very little space into which any energy analyser attachment can be placed within current SEM designs. Due to these technical limitations, only a small number of electron energy analyser attachments have proved capable of carrying out secondary electron energy spectroscopy (SEES) in the SEM [26][27][28][29] . It should be noted that the ad-hoc attempts of using in-built LVSEM SE detection systems to energy filter the SE detector signal have succeeded in providing image enhancement, but have not proved precise enough to carry out SE energy spectroscopy [30][31][32][33][34][35][36] .…”